clan join: test create vm for clan join

This adds a vm create test for the `clan join` scenario where:
  - there is no local clan to write changes to
  - a machine from a remote flake needs to be built and run
  - no users and no secrets need to be or can be managed (no flake to write files to)

pkgs/clan-cli/tests/machines/vm_without_secrets/default.nix

@@ -0,0 +1,17 @@
+{ lib, ... }: {
+  clan.networking.deploymentAddress = "__CLAN_DEPLOYMENT_ADDRESS__";
+  system.stateVersion = lib.version;
+  clan.virtualisation.graphics = false;
+
+  networking.useDHCP = false;
+
+  systemd.services.shutdown-after-boot = {
+    enable = true;
+    wantedBy = [ "multi-user.target" ];
+    after = [ "multi-user.target" ];
+    script = ''
+      #!/usr/bin/env bash
+      shutdown -h now
+    '';
+  };
+}